With the photosensitivity of the amorphous silicon, the input displays are provided with the embedded photo elements. Since the process of the amorphous silicon photo elements and the readout circuit layout of an input display are compatible with the known process of the thin film transistor array of the active matrix liquid crystal display, the manufacturing cost of the input display with embedded amorphous silicon as the photo element is more competitive than the known input display with a touch panel attached thereon.
Furthermore, the optical transmittance of the input display with the touch panel would be degraded by 20%; while the optical transmittance of the input display with amorphous silicon as the sensing devices is only dependent on the layouts of the photo sensing devices and the readout line in each pixel. Therefore, it is apparent that the input display with an amorphous silicon photo element embedded thereon is a more promising way to construct the readout pixel of the input display.
Generally, there are two typical designs of the amorphous silicon photo elements used in the input display. Please refer to FIG. 1(A) and FIG. 1(B), which respectively shows the schematic diagram of a charge-based photo element and a current-based photo element in a readout pixel of the input display. As shown in FIG. 1(A), the charge-based photo element 10 comprises a photo thin film transistor (TFT) 11, a switch TFT 12 and a capacitor C. As shown in FIG. 1(A), the activation of the switch TFT 12 is controlled by an input SW. When the switch TFT 12 is switched to on state, a current from the readout line will charge the capacitor C which is connected to the photo TFT 11 in parallel. Then, when the switch TFT 12 is switched to off state, the charge stored in the capacitor C will be discharged through the photo TFT 11. When the switch TFT 12 is switched to on state again, a current from the readout line will recharge the capacitor C back to the original charge again. Accordingly, the charge refilled to the capacitor C can be used for estimating the photo current generated by the photo TFT 11. As to the current-based photo element 20 shown in FIG. 1(B), it includes a photo TFT 21 which receives a bias voltage VBias to generate a photo current, a switch TFT 22 activated by an input SW for controlling the current to be transferred to the readout line. In such a current-based photo element, the photo current value is directly read out from the readout line.
It should be noted that both the charge-based and the current-based photo element use the photo TFT 11, 21 to generate the photo current and use the switch TFT to control the readout of the photo current. However, the current characteristics of the photo TFT between a forward-bias operation and a reverse-bias operation are asymmetric. Please refer to FIG. 2, which shows the respective characteristic curves of photo currents of a photo TFT in an illuminated state and in a non-illuminated state. As shown in FIG. 2, when the photo TFT is illuminated, the generated photo current will behave as the characteristic curve 12, which includes a forward-bias operation in a condition of the Vgs>0, which is also called on current state, and a reverse-bias operation in a condition of the Vgs<0, which is also called off current state. When the photo TFT is not illuminated, the generated photo current will behave as the characteristic curve I1, which also includes a forward-bias operation in a condition of the Vgs>0, which is also called on current state and a reverse-bias operation in a condition of the Vgs<0, which is also called off current state. Typically, the photo TFT should operates in the forward-bias state, in order to abate the signal delay resulting from the parasitic resistance and capacitance of the readout line.
Although the parasitic resistance and capacitance issue can be overcome by the forward-bias operation of the photo TFT, the readout pixel of the input display still exists a problem relating to the pixel voltage control of the readout pixel. Please refer to FIG. 3(A), which schematically shows an equivalent driving circuit in an input display according to the prior art. As can be seen from FIG. 3(A), the driving circuit 100 in each readout pixel includes a first and a second gate lines Gn-1, Gn, and a first and a second data lines Dm-1, Dm intersecting to each other, so as to form the readout pixel of the input display. Furthermore, in each readout pixel, a readout line 103 is disposed between the first and the second data lines Dm-1, Dm and passing through the readout pixel, while a common line Cp-1, is disposed between the first and the second gate lines Gn-1, Gn. Moreover, in each readout pixel, there are still two main parts, i.e. a pixel element 101 and a photo element 102 formed therein. As shown in FIG. 3(A), both the pixel element 101 and the photo element 102 are electrically connected to the common line Cp-1, through which a reference voltage is provided to a storage capacitor Cst of the pixel element 101 and through which a bias voltage is provided for driving a photo current generated by the photo element 102. Furthermore, it also can be known from the FIG. 3(A) that the pixel element 101 has a pixel TFT 1011 connected to a pixel electrode (not shown) of the input display, and the pixel electrode and a common electrode (not shown) of the input display form a liquid crystal capacitor Clc. Moreover, a further storage capacitor Cst in FIG. 3(A) is formed by the pixel electrode and the common line Cp-1.
Please further refer to FIG. 3(B), which schematically shows the operation of the driving signals according to the driving circuit of FIG. 3(A). When the first gate line Gn-1 is provided with a signal with a relatively high state, the pixel TFT 1011 of the pixel element 101 is switched on, and a signal from the first data line Dm-1 is input to the pixel element 101 and a pixel voltage Vpixel is generated thereby for providing a gray value for pixel element 101. At the same time, a switch TFT 1021 of the photo element 102 is switched on and a photo current generated by a photo TFT 1022 is output through the switch TFT 1021 to the readout line 103. Since the common voltage provided by the common line will be affected by the parasitic resistance, the voltage difference between the pixel voltage and the common voltage would be fluctuant. When the first gate line Gn-1 is provided with a signal with a relatively low state, the pixel TFT 1011 and the switch TFT 1021 are closed, and the photo current is vanished. Since the photo current is vanished, the common voltage provided by the common line will resume to a steady voltage. However, when the common voltage of the common line fluctuates again, the pixel voltage would be affected by the coupling effect. Therefore, the gray value for pixel element 101 will be affected.
Based on the above, it is the main aspect of the present invention to provide an improved driving circuit of an input display and an improved method for driving an input display, so that the voltage fluctuation issues resulting from the shared common line could be overcome.